Should Christians rejoice (Philippians 4:4) or mourn (James 4:8-10)?
Yes.
It made sense when John Piper read this out in a sermon I listened to yesterday:
A truly Christian love, either to God or men, is a humble broken hearted love. The desires of the saints, however earnest, are humble desires. Their hope is a humble hope; and their joy, even when it is unspeakable, and full of glory, is a humble broken hearted joy, and leaves the Christian more poor in spirit; and more like a little child, and more disposed to a universal lowliness of behavior. – Jonathan Edwards, The Religious Affections
Duh! Encountering the holy, glorious, infinite, indescribable God will fill you with unspeakable joy, yet make you feel so small, so dirty as to weep – in both joy and mourning. How could any true joy not leave you humbled!